Television receiver



' Feb. 13, 1940. M. MESSNER TELEVISIOI! RECEIVER Filed Feb; 29, 1936fi'ey Patented Feb. 13, 1940 UNITE S'TTES TELEVISION RECEIVERApplication February 29, 1936, Serial No. 66,429

y In Germany July 12, 1935 ,2 Claims. (01. 178-75),

The invention relates to circuits for the reception of television andhas for its object the avoidance of certain disadvantages ofarrangements of this type hitherto developed. These disadvantages andthe means by which they are avoided are explained in the followingdescription with reference to the accompanying drawing.

Figs. 1 and 2 are diagrams relating to the method of operation of thehitherto known television receiving circuits. Fig. 3 shows an excrupleof the new circuit.

It is well known to r ctify the frequency mixture of the picture signalsat suitable points, to amplify them and then convey them to the Brauntube. The amplifier situated between the rectifier and the Braun tubeis, in this arrangement, extremely complicated as this amplifier'must beable to amplify the lowest frequencies of about 25 hertz, and thehighest frequencies, nowadays 500,000 hertz, equally Well,-a requirementwhich can only be fulfilled with the employment of expensive apparatus.This method further has the disadvantage that when employing anamplifier (low frequency amplifier) inserted between rectifier andBraun. tube, and which generallycontains coupling capacities, it isimpossible to reproduce the mean picture luminosity correctly, since thecoupling capacities only transmit alternating potentials for whose meanvalue the operative point of the Braun tube must be so adjusted thatwith the picture for dark, that is, 25% of the maximum amplitude, theBraun tube is controlled just to dark. This arises from the method ofso-called gap-synchronisation in which the transmitter in theunmodulated condition operates with a carrier value of 25% of themaximum value. The picture signals are transmitted by increasing, andthe synchronising sig-' transmitting the synchronising signals thetransmitter is preferably controlled to the zero value.

In Fig. 1 the case is illustrated in which the operative point l on theBraun tube is correctly adjusted. The picture value dark modulates theBraun tube to just dark. The characteristic of the Braun tube isdesignated by K, the value M is the mean value of the alternatingpotential. The operative point is chosen exactly so that thesynchronising signals S are cut oif. If the incoming amplitude isgreater, that is, if the transmitted picture is made brighter on theaverage than in Fig. 1, then this operative point is no longer correct.Parts of the incoming frequency mixture which still contain picturemodulations render the control electrodcof the Braun tube more negativethan is required for the value dark.

These conditions are shown in Fig. 2. The parts of the picture'indicatedby hatchingare not reproduced. The operative point would hav to bedisplaced from 1 to 2.

In order to avoid this disadvantage it has been proposed to'omit theamplifier which, for example, operates in a frequency range from to500,000 cycles, and to effect the rectification in the Braun tubeitself. One modification of this circuit consists in carrying out'therectification, not in the Braun tube, but in a rectifier which 'isgalvanically coupled with the Braurf string of beads because the halfcycles of the intermediate frequency are still visible.

According to the invention an intermediate frequency is chosen which isat least five times the highest picture frequency. In accordance with afurther feature of the invention, there is employed as the amplifier,not an amplifier coupled by capacity and resistance, but an amplifierprovided with band filters. In the output of the amplifier there is alsoprovided a band filter. The Braun tube is included in the output circuitof this last band filter.

In Fig. 3 all those parts are omitted which serve to supply the feedpotentials of the apparatus such as mains apparatus and arrangements formaintaining constant the potentials supplied by the mains apparatus. Thetripping apparatus are only shown schematically as they are immaterialfor an understanding of the circuit.

The antenna l is in the input circuit of a tube 2, which for example,comprises an octode. ,3 is a generator for supplying the heterodynefrequency. The oscillations generated by 3 are mixed in the tube 2. Bythe arrangement of the multigrid tube 2 it is ensured that the generator3 does not radiate into the antenna. The interarrangement is thus verysuitable.

stages are coupled to each other by band filters 8 to I I. These filtersare similar in construction. In the output of the tube 1 is a bandfilter l2, which is of the same construction as are the filters 8 to II.Between tubes 5 and l a portion of the amplified intermediate frequencyis tapped off and conveyed to control apparatus 13 for effectingsynchronization. The tripping apparatus are designated by M and 15. Bythe tripping apparatus M the line deflection is effected, which takesplace electrostatically, and

by the tripping apparatus IS the picture deflection is effected whichtakes place electromagnetically. The control apparatus l3 contains anarrangement for segregating the synchronizing impulses from the picturesignals and also contains inverting tubes in order to obtain a controlof the tripping apparatus which is correct in phase.

The filter l2 contains directlyin its output circuit the Braun tube It..The rectification takes place in the Braun tube itself, that is, bymeans of the electrode I! which in the case illustrated is a perforatedelectrode, but a cylindrical electrode can also be employed. A Bra-untube, as is well known, insofar as its rectifying operation is concernedmay be regarded as an ordinary tube. Coupling condensers l8 and I9 arenecessary, as in the example shown the cathode of the Braun tube is at ahigh tension potential, whereas its anode is at earth potential,

as shown in the drawing. The Braun tube must therefore be blocked. indirect potential fashion to the amplifier the cathode of which must beat earth potential and the anode of which must be at high tensionpotential. Thisis efiected by the condensers l8 and 19, which forexample, are of the order of magnitude of 1000 centimeters. The capacityof the Braun tube between cathode and rectifier electrode amounts toabout 10 cm. The condensers l8 and I9'must be large with respect to theinternal capacity of the Braun tube in order to convey as much aspossible of the high frequency potential (intermediate frequencypotential) to the Braun tube. In the example shown the potential loss,as will be seen from the capacity values, amounts to 1%. The

The condensers l8 and I9 must be made to stand several thousand voltspotential, a requirement which quency iron cores.

can easily be fulfilled. The Braun tube is included in the outputcircuit of the filter l2 by the fact that the condensers l8 and I!)which are in series with the internal valve capacity are connected inparallel to the condenser C of the filter H.

The condensers C are adjustable in order to enable the output circuitsof the band filters to be tuned.

The use of an amplifier, coupled by band filters, for the intermediatefrequency has the advantage that a Very high intermediate frequency canbe employed without difiiculty. The high intermediate frequency is, asmentioned before, favourable for the picture quality since after therectification uniform picture lines are formed, that is, not picturelines which are like a string of beads. The construction of the bandfilter moreover, is simplified by the use of the high intermediatefrequency as in television very wide frequency bands have to betransmitted. The higher the intermediatefrequency, the smaller is therelative band width, that is, the more simply can the filters beconstructed.

The filters 8 to 12 consist in the well known manner of self inductancecoils A with high fre- In addition, however, ohmic resistances areplaced in parallel with the condenser C and the coils A in order toobtain sufficient attenuation, that is, a sufficient band width.

What is claimed is:

i. In a television receiver, means for producing an intermediatefrequency which is at least five times the highest picture frequency,means for amplifying said intermediate frequency, said amplifying meanscomprising a multistage amp-lifier provided with band filters forcoupling said stages, a Braun tube, and a band filter interconnectingthe output circuit of said amplifying means and the Braun tube, thistube being included in the output circuit of said last named bandfilter.

2. In a television receiver according to claim 1, coils forming part ofthe band filters and provided with cores of high frequency iron, and attenuation resistances connected in parallel with these coils.

MAXIMILIAN MESSNER.-

